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| Fig. 1: The nuclear plants at Three Mile Island in 1979. (Source: Wikimedia Commons) |
Ground-based robots are useful for investigating harsh environments. One of the most important applications is accessing places with extremely high radiation densities or toxic/combustible atmospheres harmful to human health. The recent creation of robotic systems capable of working in nuclear environments has now made it possible to safely perform a wide range of human-level work tasks including radiation environment inspections, maintenance, analysis, and repair. [1]
Historical nuclear reactor accidents raised the urgency of developing remotely operated robots to explore and fix damaged reactors. One such accident was the 1979 nuclear meltdown at Three Mile Island, shown in Fig. 1. But the much more disastrous accident that took place seven years later at Chernobyl drove home the need for technically improved radiation-resistant robots that could work in environments instantly lethal to humans.
Operating robots in radioactive environments is not trivial, as complementary metal-oxide semiconductors, flash memories, etc. are radiation-sensitive due to discharge of the floating gates by γ-rays. Thus, robots often cannot function for an adequate amount of time in conditions of extreme radioactivity. The inadequacies of remote-based robot technology became particularly clear in 2011 during the Fukushima accident. [2] This triggered new research efforts to improve the capabilities of robots and artificial intelligence used in nuclear clean-up. Important new features included the ability to execute simple tasks such as navigation and environment analysis.
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| Fig. 2: Packbot robot for conducting radiation mapping and cleaning. (Source: Wikimedia Commons) |
The Fukushima Daiichi nuclear power plant accident occurred in 2011 [2]. It was caused by 50 foot tsunami waves generated by a 9.0 earthquake 80 miles off the coast of Japan. The damage cost of this disaster was approximately $202.5 billion in 2016. [2,3] Due to extremely high radiation levels, only robots could go inside the reactors to assess damage.
A number of different robots were used to fix and analyze the damaged nuclear power plant. The JAEA-3 was used to observe γ-ray imaging on many floors of the nuclear reactor buildings. The major robot type for Fukushima accident was the reconnaissance type. [1] Two robots, Packbots (Fig. 2) and TALON, are used to undertake reconnaissance tasks such as radiation mapping and cleaning. The JAEA-3 got redesigned to operate safely under high radiation conditions. [4] Between 2012 and 2014, the Four-Legged Walking Robot and the Survey Runner were used to investigate the torus room in Units 2 and 3 at Fukushima nuclear plant. The Four-Legged Walking Robot was designed to maneuver in the small and narrow spaces on the top of the suppression chamber and the vent pipes. The Survey Runner was used to investigate the upper part of the suppression chamber and to approach underwater locations to look for leaking spots using a camera.
In 2013, Frigoma was used to harvest information about the ambient radiation dose rate as well as scanning the area near the Primary Containment Vessel (PCV). Also, more advanced applications including a novel γ camera with pan/tilt and shutter mechanisms and mounted on a tracked wheel vehicle were developed. PMORPH and SCORPION were employed to observe the interior of the PCV of Unit 1, where the radiation levels ranged from 3 to 17 Svh-1. [1] The SCORPION operation failed early on due to exposure to the extremely high radiation level of 650 Svh-1. [1]
There are many factors to consider developing robots. The representative robotic components include mechanical structure, environmental sensing systems, self-diagnostics, communications and power supplies, and electronics. The novel functionalities and system designs that have been developed include
Accommodating a narrow path with sharp corners. [5]
Positioning sensors for accurately stopping the vehicle at the inspection position. [6]
The ability to manipulate varying loads up to 0-25 kg [7]
The ability to sample floor sludge [8]
The ability to inspect valve orientation [9]
The use of optical wheel encoders and proximity sensors staggered for different depths to determine passing doorways or external architecture [10]
Obstacle avoidance using sonar and infrared sensors [11]
The use of eddy currents to locate welds in pipes [12]
Collision-free navigation down a narrow aisle of storage drums [13]
The ability to drag a 100kg payload horizontally [14]
The ability to cross 400 mm wide trenches [15]
Multiplexing to keep the umbilical link flexible and light [16]
Direction compensation using a gyroscope sensor [17]
The ability to sense oxygen and hydrogen density [18]
Non-wireless communication was required. [19] All the features cannot be integrated into a single robot. There are trade-offs between capabilities and mission goals that dictate what features are appropriate for a given design. For instance, there could be circumstances in which a sacrificial robot needs to be employed to discover a highly radioactive environment where the robots need to investigate the large area of interest. Hence, a low cost working prototype design is required for the robot. For reusable robots, sensors and electronics need to be heavily shielded to prevent the radiation from inducing errors in instrumentation measurements. Therefore, more expensive yet robust electronic system designs are required. Designing robots requires various parameters to consider and employ; however, functional components, implementations, and performance are considered as a general categorization to design the robots for a particular purpose.
© Mun Sek Kim. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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[12] H. T. Roman, B. A. Pellegrino, and W. R. Sigrist, "Pipe Crawling Inspection Robots: an Overview," IEEE Trans. Energy Convers. 8 576 (1993).
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[14] B. L. Luk et al., "Robug III: a Tele-Operated Climbing and Walking Robot," UKACC International Conference on Control '96, IEEE 651404, 2 Sep 96.
[15] J. Perret, "Service Robots for Nuclear Safety: New Developments by CYBERNETIX," 1998 IEEE International Conference on Robotics and Automation, IEEE 680631, 20 May 98.
[16] J. De Geeter, M. Decréton, and E. Colon, "Challenges of Telerobotics in a Nuclear Environment," Rob. Auton. Syst. 28, 5 (1999).
[17] Y. Hosoda et al., "SWAN: A Robot for Nuclear Disaster Prevention Support," Adv. Robotics 16, 485 (2002).
[18] Y. Yuguchi and Y. Satoh, "Development of a Robotic System for Nuclear Facility Emergency Preparedness Observing and Work-Assisting Robot System," Adv. Robotics 16, 481 (2002).
[19] K. Nagatani et al., "Redesign of Rescue Mobile Robot Quince," 2011 IEEE International Symposium on Safety, Security, and Rescue Robotics, IEEE 6106794, 1 Nov 11.
